Gunay Paksoy of the Istanbul Archaeology Museum believes the writing is either a letter or part of an amulet. "There are four names, Ahmad, Mohammad, Amir and Mawali in the text" he notes, adding that the first three lines may be part of a phrase from the Quran.
The Marmaray project is an attempt to build an undersea rail tunnel linking the European and Asian sections of Istanbul, running under the Bosphorus strait. The project began in 2004, but has experienced numerous delays because of the large amount of archaeological discoveries made while digging. This includes remains from the 4th-century port of Theodosius, traces of the city wall of Constantine the Great, and the remains of several ships. Two other fragments of Arabic text have also been discovered by archaeologists, but were unreadable.
The document may come from the episode when Constantinople was besieged for a second time by Arab forces in the years 717-718. The siege, first led by Umayyad Caliph Sulayman ibn Abd al-Malik and then by his successor Umar II, was a major attempt at taking the Byzantine capital using land and naval forces. Unable to breach the city walls and being attacked by the Byzantine allies the Bulgars, the Arab forces retreated - the loss being one of the causes of the collapse of the Umayyad Dynasty.
Sources: Izmirde Yasim, World Bulletin